1. Field of the Invention
The present invention relates to an aspherical zoom lens. More particularly, the present invention relates to a high-performance aspherical zoom lens for use in a 3-CCD (charge coupled device) type video camera or the like, which has a high zoom ratio and a long back focal distance.
2. Description of the Related Art
In response to recent demands for excellent operational efficiency, good mobility and high image quality in video cameras, high-resolution and compact imaging devices are becoming a mainstream of imaging devices. Meanwhile, in this connection, high-performance and high-magnification zoom lenses which have a large aperture ratio, are compact in size and light in weight are in strong demand. Furthermore, due to a big demand for the reduction of production costs of zoom lenses, high-performance and high-magnification zoom lenses in which the number of lens components is reduced are in urgent need.
However, in known high-magnification zoom lenses, not only the diameters and overall length of lenses become large in order to realize the high magnification, but a large number of lenses are required to be used for performing more strict aberration corrections. As a result, conventional video cameras including the zoom lenses become larger, heavier, and more expensive and therefore, the zoom lenses have not been suitable for use in video cameras for home use.
Hereinafter, an exemplary zoom lens for use in a video camera is described with reference to FIG. 2 (for example, see Japanese Patent Application No. 3-232840).
FIG. 2 is a view showing the structure of a conventional zoom lens 200 for use in a video camera. As is shown in FIG. 2, the zoom lens 200 includes a first lens group 21 acting as a light converging portion, a second lens group 22 acting as a magnification changing portion, a third lens group 23 acting as a light converging portion, a fourth lens group 24 acting as a focusing portion, a color separation optical system 25, and a glass plate 26 which is optically equivalent to a quartz crystal filter, a face plate of an imaging device, and the like. The reference numeral 27 denotes an image plane.
The operation of the zoom lens 200 having the above-described arrangement is now described. The first lens group 21 is fixed at a predetermined position relative to the image plane 27, and has an image forming function. The second lens group 22 is movable on an optical axis so as to change magnification so that a focal length of an entire system is changed. The third lens group 23 is also fixed at a predetermined position relative to the image plane 27, and has a function of converging divergent light produced by the second lens group 22. The fourth lens group 24 is movable on the optical axis and has a focusing function. Variations in position of the image plane due to travel of the second lens group 22 at the time of zooming are eliminated by displacing the forth lens group 24. Thus, the image plane is fixed at a predetermined position.
In the above-described conventional zoom lens 200, the fourth lens group 24 consists of three single lenses. This results in a strict and narrow tolerance for the zoom lens system. Accordingly, in cases where inclination of lens occurs, or in other cases, it is difficult to maintain the performance of the zoom lens system. In addition, if the zoom ratio is set to about 10, and a back focal distance is ensured to be long in order to realize a high magnification, it becomes difficult to perform aberration correction over an entire zoom range and an entire shooting distance, so that high image quality cannot be achieved.